关键词: 渤海海域南部, 中生界, 火山岩, 潜山, 储层特征, 控制因素

Abstract: Mesozoic volcanic rocks are widely developed in the Bohai Sea area, demonstrating substantial hydrocarbon exploration potential. The southern Bohai Sea is one of the key areas for the exploration of volcanic rock reservoirs. The Mesozoic volcanic rocks exhibit diverse lithologies, significant spatial differences in reservoir physical properties and hydrocarbon enrichment levels, and unclear formation mechanism of high-quality reservoirs. These issues severely restrict the next step of volcanic reservoir exploration in this area. By comprehensively utilizing the drilling/logging data, core and rock thin section petrography, scanning electron microscopy observations, and geochemical element analysis, an in-depth study was conducted on the characteristics and controlling factors of the Mesozoic volcanic reservoirs in southern Bohai Sea. The research results indicate that the volcanic rocks in southern Bohai Sea are dominated by intermediate-basic types, followed by intermediate-acidic volcanic rocks. The main lithofacies are effusive lava and explosive pyroclastic rocks. The reservoir space is predominantly composed of secondary dissolution pores and fractures, exhibiting high porosity (averaging 14.0 %) but low permeability and complex pore throat structure. The dominant lithology-lithofacies assemblage serves as the material basis for the development of high-quality reservoirs, wherein the intermediate-acidic volcanic reservoirs exhibit superior quality to the intermediate-basic volcanic rocks, and the explosive pyroclastic rock reservoirs outperform effusive volcanic rocks in reservoir qualities. The weathering and leaching processes control the vertical zonation of reservoir properties, with enhanced reservoir characteristics observed within 160 meters of the top of the weathering crust. Diagenetic evolution governs the lateral heterogeneity in reservoirs. The pore evolution models of volcanic reservoirs in the study area are divided into three types, i.e., the early strong filling with late weak dissolution model (Type 1), the early strong filling with late strong dissolution model (Type 2), and the early weak filling with late strong dissolution model (Type 3). Type 3 exhibits the best reservoir physical properties. Fractures are a key factor determining the reservoir effectiveness. NNE-trending strike-slip faults and nearly EW-NWW-trending extensional faults jointly control the development of reservoir fractures. The optimal fracture effectiveness is observed in later-stage fractures that oriented parallel to the contemporary maximum horizontal stress, with favorable temporal coupling to hydrocarbon charging episodes. The study proposes a wellbore trajectory offset method for the rapid evaluation and prediction of reservoir geostress and effective fracture development. Comprehensive research suggests that in the areas where intermediate-acid eruptive volcanic rocks are developed along the Tan-Lu strike-slip fault zone, key areas for the development of high-quality volcanic reservoirs include the zones with medium to high porosity within 160 meters vertically below the weathering crust, and those exhibiting the horizontal combination of favorable diagenetic evolution patterns and well-developed effective fractures.

Key words: southern Bohai Sea, Mesozoic, volcanic rock, buried hill, reservoir characteristics, controlling factors